Lanthanide ion extraction by trifluoromethyl-1,3-diketonate-functionalised ionic liquids adsorbed on silica

While acetylacetone (acacH) derivatives are, upon deprotonation, ubiquitous ligands in coordination chemistry, their potential to form stable ionic liquids has not been studied so far. Here we describe a straightforward synthesis of novel trifluoroacetylacetone-functionalised imidazolium salts. These salts are built from an imidazolium ring substituted on one side with a flexible chain of fixed length carrying a terminal acacH group and on the opposite side a paraffin chain of various lengths. By changing the length of these flexible chains (n=4, 8, 12) and the nature of the counter-anions (PF(6)(-), BF(4)(-), NTf(2)(-)), room-temperature ionic liquids were produced. Their application for the extraction of lanthanide salts (Eu, Tb) from dilute aqueous solution has been investigated. The presence of a strong UV absorber (imidazolium ring, λ(abs)=290 nm) allows photosensitisation of the Eu(III) and Tb(III) luminescence by efficient energy transfer, and thus extraction of these two lanthanides can be followed by fluorescence techniques. It appears that loading of the ionic liquids onto silica particles pre-treated with a dilute aqueous solution of NaOH is the most efficient way to obtain fast and almost quantitative (>99.9%) extraction of the metal ions as their diketonato complexes. The procedure is reproducible and the loaded SiO(2) particles can be simply treated with acid to strip the metal ions and regenerate the adsorbed (protonated) extractant.